1. Field of the Invention
The present invention relates to methods for alginate production using mucoid bacterial cultures. The present invention also provides for pharmaceutical compositions that contain bacterial alginate. The bacterial alginate may be produced using genetically engineered bacteria.
2. Background Art
Alginate
Alginates are salts of alginic acid, which is a linear hetero-polysaccharide. Alginates are comprised of two subunits, β-D-mannuronic acid (denoted M units) and α-L-guluronic acid (denoted G units). Alginates may be found in and isolated from various species, in particular from algae belonging to the order Phaeophyceae and soil bacteria such as Azotobacter vinelandii and Azotobacter crococcum. Common algal sources of alginates include Laminaria digitata, Ecklonia maxima, Macrocystis pyrifera, Lessonia nigrescens, Ascophyllum nodosum, Laminaria japonica, Durvillea antartica, Durvillea potatorum and Laminaria hyperborea. 
Alginates produced from various sources differ considerably in their structure. For example, alginates produced by seaweed are generally not acetylated, whereas bacteria produce alginates with a higher degree of acetylation. In addition, the molecular weight and the ratio of β-D-mannuronic acid and α-L-guluronic acid units in alginates can vary based on the alginate source and the conditions in which the alginate is produced. These structural differences can result in changes in alginate properties.
Alginates are commonly purified from brown seaweeds. However, brown seaweed is a limited resource and extraction of alginate results in destruction of this precious resource. In addition, there are several problems associated with seaweed alginate. First, their harvest is seasonal and alginate production is dependent on cold ocean temperature, which is rising, most likely due to global warming. Second, extraction of seaweed alginate involves as many as 15-20 different processing steps. Third, the composition of the alginate produced by the seaweed is fixed and cannot be altered to produce a better or different product to expand commercial applications.
Alginate can be used in a wide variety of products. For example, seaweed alginates are used in food, dental and cosmetic products. The alginates are particularly useful as gelling, thickening, stabilizing, swelling and viscosity imparting agents. Seaweed alginate is used in the textile and paper industries and also serves as a thickening agent in common food items, such as ice cream, salad dressing, pet food chunks, low fat spreads, sauces and pie filings. Seaweed alginate is also incorporated into wound dressings to provide a moist surface for healing. Alginate fibers trapped in a wound are readily biodegraded. Dressings with seaweed alginate are used to treat ulcers in diabetic patients. Propylene glycol alginate has been used as an acid-stable stabilizer for uses such as preserving the white fluffy head of foam on beers. Seaweed alginate absorbs radioactive elements, heavy metals and free radicals. Because alginate cannot be broken down by bile or saliva and cannot be absorbed by the body, it is secreted from the body together with the heavy metals and radioactive substances. The ever-increasing applications of this biopolymer have led to continued interest in better understanding the biosynthesis pathway and regulatory mechanisms as well as optimization of microbial production process.
Regulation of Alginate Production Pathway in Pseudomonas 
Synthesis of alginate and its regulation has been the object of numerous studies (Govan, J. R., and V. Deretic, Microbiol. Rev. 60:539-74 (1996); Ramsey, D. M., and D. J. Wozniak, Mol. Microbiol. 56:309-22 (2005)). Alginate production is positively and negatively regulated in wild-type cells of Pseudomonas. 
Three tightly linked genes algU, mucA, and mucB have been identified with a chromosomal region shown by genetic means to represent the site where mutations cause conversion to mucoidy (see U.S. Pat. Nos. 6,426,187, 6,083,691, 5,591,838, and 5,573,910, incorporated herein by reference in their entireties).
Positive regulation centers on the activation of the alginate biosynthetic operon (Govan, J. R., and V. Deretic, Microbiol. Rev. 60:539-74 (1996)). Positive regulators include the alternative stress-related sigma factor AlgU (Martin, D. W., et al., Proc. Natl. Acad. Sci. 90:8377-81 (1993)), also called AlgT (DeVries, C. A., and D. E. Ohman, J. Bacteriol. 176:6677-87 (1994)), and transcriptional activators AlgR and AlgB, which belong to a bacterial two component signaling system. The cognate kinase of AlgB is KinB (Ma, S., et al., J. Biol. Chem. 272:17952-60 (1997)) while AlgZ (Yu, H., et al., J. Bacteriol. 179:187-93 (1997)) may be the kinase that phosphorylates AlgR. However, unlike a typical two-component system, alginate overproduction is independent of phosphorylation of AlgR or AlgB (Ma, S., et al., J. Bacteriol. 180:956-68 (1998)).
Negative regulation of alginate has focused on the post-translational control of AlgU activity. In alginate regulation, the master regulator is AlgU and the signal transducer is MucA, a trans-inner membrane protein whose amino terminus interacts with AlgU to antagonize the activity of AlgU, and the carboxyl terminus with MucB, another negative regulator of alginate biosynthesis. The algUmucABC cluster is conserved among many Gram-negative bacteria. AlgU belongs to the family of extracytoplasmic function (ECF) sigma factors that regulate cellular functions in response to extreme stress stimuli. The action of ECF sigma factors is negatively controlled by MucA, MucB and MucC. This set of proteins forms a signal transduction system that senses and responds to envelope stress.
MucA is the anti-sigma factor that binds AlgU and antagonizes its transcriptional activator activity (Schurr, M. J., et al., J. Bacteriol. 178:4997-5004 (1996)). Consequently, inactivation of mucA in P. aeruginosa strain PAO1 results in the mucoid phenotype (Alg+) (Martin, D. W., et al., Proc. Natl. Acad. Sci. USA 90:8377-81 (1993); Mathee, K., et al., Microbiology 145:1349-57 (1999)). Clinical mucoid isolates of P. aeruginosa carry recessive mutations in mucA (Anthony, M., et al., J. Clin. Microbiol. 40:2772-8 (2002); Boucher, J. C., et al., Infect. Immun. 65:3838-46 (1997)). The transition from a non-mucoid to mucoid variant occurs in concurrence with the mucA22 allele after exposure to hydrogen peroxide, an oxidant in neutrophils (Mathee, K., et al., Microbiology 145:1349-57 (1999)).
MucB is located in the periplasm in association with the periplasmic portion of MucA (Mathee, K., et al., J. Bacteriol. 179:3711-20 (1997); Rowen, D. W., and V. Deretic, Mol. Microbiol. 36:314-27 (2000)). MucC is a mild negative regulator whose action is not completely understood, but thought to be in synergy with MucA and/or MucB (Boucher, J. C., et al., Microbiology 143:3473-80 (1997)). MucD is a negative regulator whose dual functions include periplasmic serine protease and chaperone activities that are thought to help remove misfolded proteins of the cell envelope for quality control (Boucher, J. C., et al., J. Bacteriol. 178:511-23 (1996); Yorgey, P., et al., Mol. Microbiol. 41:1063-76 (2001)).
Alginate Production in Pseudomonas 
Alginate production in mucoid strains of P. aeruginosa has been limited because these strains quickly convert to non-mucoid strains and do not produce sufficient amounts of alginate for commercial application. Other species of Pseudomonas generally produce small amounts of alginates, or alginates of low molecular weight. In spontaneous alginate-producers, non-mucoid revertants tend to arise frequently (Flynn and Ohman, J. Bacteriol. 170:1452-1460 (1988)).
Non-pathogenic species of Pseudomonas such as P. putida, P. mendocina and P. fluorescens produce exopolysaccharides similar to acetylated alginates. (Govan J. R. W. et al., J. of General Microbiology 125:217-220 (1981)). Conti et al. also describe production of alginates from P. fluorescens and P. putida. (Conti, E. et al., Microbiology 140:1125-1132 (1994)). However, these strains produce small quantities of alginate.
There is therefore a need for suitable bacterial sources and methods for inexpensive mass production of alginate. In particular, there is a need for bacterial sources producing large amounts of high quality alginate with defined structure and desired molecular weight.